﻿﻿Most herbicides break down quickly (within days or weeks) when exposed to sun, heat, and soil microbes. The compost process, with its fluctuating temperatures and thriving microbial activity, also does a great job of degrading most herbicides, pesticides, and other chemicals. However, there is one class of herbicides that may persist up to several years in soil, livestock manure, mulch, and compost. Therefore it is important for gardeners to be aware of how to prevent accidental damage to sensitive plants. These include peas and beans, carrots, tomatoes, peppers, spinach, and strawberries.

Herbicides that contain the active ingredients of aminopyralid, clopyralid, picloram, or aminocyclopyrachlor have the potential to accumulate in manure, compost, or soil. There are many products available on the market that contain one or more of these active ingredients so be sure to read the product label thoroughly!

These herbicides act as growth regulators and are very effective at killing many broad-leaf plants. For this reason they are commonly used on lawns, pastures, and hay fields. What makes these herbicides especially unique is their persistence in livestock manure. When an animal consumes the hay or grass from an area where these herbicides have been used, the active ingredients (clopyralid, etc.) do not break down in the gut and are excreted with the manure. In addition, they also persist in the soil, stockpiled manure, or compost.

When used on lawns, accidental damage can occur if the grass clippings are used as mulch or for composting. Straw bales can also be problematic if from a field sprayed with one of these herbicides.

Growth regulator herbicide damage on beans, sunflowers, and tomatoes

Regardless of active ingredients, carefully read and follow all labels! For more information on safe and effective pesticide use, see my blog post on the topic.

How can you reduce the risk of residual herbicides in the garden?

Do not use products containing the above listed active ingredients on your lawn if you plan to compost the grass clippings or use them as mulch.

If you are using livestock manure in your garden (even after composting) find out what herbicides were used on the fields where the animals grazed or where the hay was grown.

When using straw for mulch, or a straw-bale garden, find out what herbicides were used.

Conduct a simple bioassay for herbicide residues using the instructions provided here.